This invention relates to input interface circuits for removing noise from input signals supplied to electronic circuit components such as microcomputers which control an automotive engine, etc.
FIG. 5 is a circuit diagram showing the organization of a conventional input interface circuit for an automotive microcomputer (CPU), and FIG. 6 shows waveforms generated within the circuit of FIG. 5. In accordance with the states of the automobile, switches 1 are turned on and off. The resulting signals are inputted to the input interface circuit 12 via the input terminals 2. Each channel for the input signals includes: a pull-down resistor 3a or a pull-up resistor 3b; a resistor 4 and a capacitor 28 constituting an RC filter; diodes 5 and 6 for providing protection against input voltage surges; resistors 29 and 30; a transistor 31 for waveform shaping; and a pull-up resistor 32.
The operation of the input interface circuit for the upper input channel is as follows. When the switch 1 is turned on to turn on the transistor 31, the input to the CPU (microcomputer) 13 goes from the high to the low state. In response thereto, the CPU (microcomputer) 13 outputs an appropriate control signal for the automobile.
When an input waveform as shown at FIG. 6(a) is inputted to the terminal 2 of the upper channel of the interface circuit 12, the waveform is shaped via the RC filter and the surge protection diodes into a waveform shown at FIG. 6(b), which is inputted to the base of the transistor 31. In response thereto, the transistor 31 outputs a shaped waveform shown at FIG. 6(c), which is inputted to the CPU (microcomputer) 13.
The operation of the lower channel is similar except for the polarity of the signals.
The above conventional input interface circuit, however, has the following disadvantage. The characteristics of the RC filter vary due to the variations of the components or in accordance with the variation of the ambient temperature. Thus, the capacity of the capacitor 28 must be designed at a large enough value which allow for a sufficient margin for such variations of the filter characteristics. Larger capacitors not only push up the production cost, but also occupy larger spaces on the substrate board of the interface circuit 12, which makes the high density mounting of the components very difficult.
For the purpose of increasing the mounting density of components, the whole interface circuit 12 may be formed as an integrated circuit. It is feasible, however, in the case of an analog system of as shown in FIG. 5, to produce the interface circuit 12 as a monolithic integrated circuit where the values of the resistances and capacitances are large. On the other hand, although the interface circuit 12 can be formed as a hybrid integrated circuit by mounting the components on a ceramic board, this method of production increases the cost and the number of steps required for the quality control.